The structure of ant assemblages on Amazonian fluvial islands: the roles of recurrent disturbance and isolation.

Disturbance and dispersal processes jointly shape assemblage structures across multiple spatial scales. The flood pulse in Amazon rivers is a large-scale natural and seasonal disturbance that affects floodplain forests and fluvial islands. We evaluated how flooding and isolation of fluvial islands act as environmental filters, structuring taxonomic and functional ant assemblages in the Amazon. During the dry season, we collected ground-dwelling and arboreal ants in 12 transects in terra-firme (non-flooded forest), 12 in igapó (flooded forest connected with terra-firme), and 11 in fluvial islands (flooded forest isolated from terra-firme and igapó) in Anavilhanas National Park, Amazonas, Brazil. We recorded 205 ant species and morphospecies. The arboreal ant richness was similar between the environments, but terra-firme forest harbored higher richness in the ground stratum. Ant composition strongly differed between the environments, and the turnover dominates the beta diversity patterns in both the ground and arboreal strata. Cryptobiotic attines, ground-dwelling specialist predators, and raid-hunting predators were extremely rare or absent, while arboreal predators and generalist omnivores were relatively more abundant in fluvial islands. Flooding and spatial isolation were related to ant assemblage structure promoting strong species turnover in fluvial islands. Ant species that exclusively live on the ground can only survive where the influence of floods is minimal because it reduces nesting sites and availability of food resources. Since ant composition is markedly different between environments, fluvial islands contributed to regional turnover in ant species diversity, increasing the conservation value of this habitat at larger scales.